The 3rd Generation Partnership Project (3GPP) is responsible for the standardization of the UMTS (Universal Mobile Telecommunication Service) system, and LTE (Long term Evolution) is now under discussion as a next generation mobile communication system of the UMTS system. LTE is a technology for realizing high-speed packet-based communication that can reach a data rates of about 100 Mbps on the downlink and about 50 Mbps on the uplink.
The 3GPP LTE system will include protocols intended to minimize the loss of data transmitted through the radio link between entities in the network. One of these protocols is known as the radio link control (RLC) protocol used for communication between user equipments in a cell and a controlling node or nodes, e.g. a radio base station(s) of the cell. In LTE, a base station is also known as eNB (enhanced/evolved NodeB) or eNodeB which performs the functions of a conventional radio access network (RNC) node of a UMTS Node B. In addition, eNBs in LTE will interact directly with the core network and with other eNBs
The RLC protocol can operate in different modes, e.g. a acknowledged mode (AM) and a unacknowledged mode (UM). In LTE, the RLC is used to cover for residual errors caused by the lower layer protocol, when run in AM mode, and to perform in-sequence delivery of protocol data units (PDUs) also when run in UM mode.
The in-sequence delivery of PDUs is required, because the lower layer in LTE uses Hybrid automatic resend request (HARQ) protocol, which may cause the data to be delivered to RLC in a different order then in which it was originally given to the lower layer. HARQ is generally used for facilitating fast error detection and correction. HARQ is known as a stop and wait protocol; subsequent transmission can take place only after receiving ACK/NACK (acknowledgment(s)/negative-acknowledgement(s)). In case an ACK is received a new transmission is done else a retransmission is done. ACK indicates to the transmitting side that data has been correctly received. NACK indicates to the transmitting side that data has not been correctly received i.e. received only partly or not at all.
As mentioned earlier, the RLC supports the RLC UM mode. For each PDU intended to be transmitted, a PDU header includes a sequence number such that the receiving side knows which PDU had been lost during transmission. When considered from the transmitting side, if the transmitting side operates in UM mode, it does not check whether the receiving side correctly received the corresponding PDU, and once a PDU is transmitted, it is not re-transmitted. When considered from the receiving side operating in RLC UM mode, the PDUs that have been missing are determined by referring to the sequence numbers of the received PDUs, and for those PDUs determined to be missing, waiting is performed only until a special re-ordering timer expires. PDUs are thus delivered in-sequence as long as they are received while the re-ordering timer is running. When the re-ordering timer expires, out-of-sequence delivery is performed, and the missing PDUs are considered lost.
The RLC AM mode includes the sending of PDUs over the radio interface carrying higher layer data, reordering of those PDUs in the receiving side, detecting lost PDUs and requesting retransmissions for those PDUs that are considered lost. Each PDU is also here identified using a sequence number (SN). Therefore, the reordering of data PDUs is performed based on the SN which is included in the RLC header. The retransmissions of lost PDUs are requested by the receiving side in a special control PDU, called the RLC status PDUs, status PDU, status PDU report, status report or simply a STATUS. The status PDU is sent from the receiving side to the transmitting side. The status PDU indicates ACKs concerning correctly received data. The status PDU also indicates NACKs concerning data that has not been correctly received, i.e. received only partly or not at all. The ACKs and NACKs in the RLC status PDU are thus sent as PDU sequence numbers in order to identify the corresponding PDUs in question.
Contrary to a data PDU, the status PDU is not numbered, i.e. it does not include a SN. Therefore, the status PDU cannot be reordered in the same way as the data PDUs. The lack of reordering for the status PDUs in RLC causes them to be received sometimes out of order. This means that the status PDUs arrive to the RLC transmitting side in an order which is different from the order in which they where transmitted from the RLC receiving side. In the prior art RLC specification described in the 3GPP technical specification TS 36.322 version 8.1.0, it is specified that a RLC transmitter should re-transmit PDUs that are marked as missing in a status report. However, this prior art fails in describing the action to be performed in case an out of order reception of a status PDU is observed. A drawback with having an out of order reception of a status PDU is that it leads to that a previously acknowledged PDU can be later negatively acknowledged by an older status PDU. This will lead to unnecessary retransmission of previously acknowledged PDUs. Furthermore, unnecessary retransmissions result is a waste of radio resources and an increase in interference in the network.